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Novel Process for the Preparation of Serinol

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Novel Process for the Preparation of Serinol Europaisches Patentamt 0 348 223 J European Patent Office fin Publication number: A2 Office europeen des brevets EUROPEAN PATENT APPLICATION Application number: 89306393.3 int. a*: C 07 C 89/00 C 07 C 91/12 Date of filing: 23.06.89 Priority: 23.06.88 US 210945 ® Applicant: W.R. Grace & Co.-Conn. 1114 Avenue of the Americas New York New York 1 0036 (US) Date of publication of application: 27.12.89 Bulletin 89/52 @ Inventor: Quirk, Jennifer Maryann 6566 River Clyde Drive Designated Contracting States: 20777 (US) AT BE CH DE ES FR GB GR IT LI LU NL SE Highland Maryland Harsy, Stephen Glen 16236 Compromise Court Mt. Airy Maryland 21771 (US) Hakansson, Christer Lennart Vikingsgaten 3 S-25238Helsingborg (SE) @ Representative: Bentham, Stephen et al J.A. Kemp & Co. 14 South Square Gray's Inn London WC1R5EU (GB) @ Novel process for the preparation of serinol. (g) A process for forming 2-amino-1 ,3-propanediol by reduc- ing a 5-nitro-1,3-dioxane and subsequently hydrolyzing the reduced compound. CO SI 00 3 Q_ LU Bundesdruckerei Berlin EP 0 348 223 A2 Description NOVEL PROCESS FOR THE PREPARATION OF SERINOL The present invention relates to a novel process to form 2-amino-1 ,3-propanediol (commonly known as "serinol"). The present process provides a means of forming serinol using readily formed materials under mild 5 and easily handled conditions suitable for industrial application. Serinol is a highly desired material required for the preparation of nonionic x-ray contrast media, such as iopanediol (N,N'-bis[2-hydroxy-1-(hydroxymethyl)ethyl]-2,4,6-triiodo-5-lactamidisophthalamide). Serinol has been previously prepared from 2-oximino-1 ,3-propanediol, 2-nitro-1 ,3-propanediol, serine, serine methyl ester, or oximinomalonic acid diethyl ester. In most cases the required processes provided low 10 yields and, in certain instances, utilizes poorly accessible starting materials. In addition, the processes normally entail the generation of decomposable and dangerous imtermediates which require special equipment and handling practices. The expense of the reactants and equipment required, as well as the special handling needed leads to unsatisfactory processes for industrial application. The major commercial method of producing serinol is disclosed in U.S. 4,448,999 and involves an 15 improvement of a process disclosed in DE 2,742,981. The process requires the initial formation of a solid product, sodium nitro-1 ,3-propanediol. The diol must be removed from the reaction zone in order to minimize unwanted further reaction to the triol. The diol is then subjected to hydrogenation at elevated pressures of 50 bars or greater in the presence of palladium on carbon as the hydrogenation catalyst. U.S. '999 discloses that the hydrogenation process should be carried out under stringent temperature conditions to achieve 20 consistently good yields. The difficulty with this method is the need for isolating and utilizing sodium nitro-1 ,3-propanediol which is known to be an unstable material which decomposes with catastrophic results. An alternate method for producing serinol is disclosed in DE 2,829,916. This process involves the reductive amination of 1 ,3-dihydroxyacetone. Due to the difficulty in synthesizing the required starting ketone, this process is not economically competative and is not widely used on an industrial scale. 25 It is highly desired to have a process capable of forming serinol which utilizes readily available and easily handled materials. Summary of the Invention 30 The present invention is directed to a process which is readily adaptable to industrial application and utilizes reactants and conditions which do not present a handling problem. The instant process comprises hydrogenation of certain 5-nitro-1 ,3-dioxanes under mild conditions to form the corresponding amino derivatives and hydrolyzing the amino derivatives to give the desired serinol. 35 Detailed Description of the Invention The present process provides the desired serinol using readily available reactants under conditions easily 40 adaptable for industrial application. The total synthesis can be accompanied by the following reactions: 1. Nitromethane is reacted with three moles of formaldehyde to form tris (hydroxymethyl)nitromethane (I). CH NO, + 3CH_O >C-(CH_OH)- 45AC I NO2 (I) This Henry Reaction is carried out by contacting the nitromethane and formaldehyde in a solvent normally selected from a lower alkyl alcohol or water (preferably methanol) in the presence of a catalytic amount of base SO such as sodium or potassium hydroxide. The formaldehyde should be present in at least stoichiometric amounts based on nitromethane (i.e. 3 moles of formaldehyde per mole of nitromethane). This reaction is known and the product can be commercially obtained. This product, unlike the dihydroxymethyl nitromethane sodium salt used in U.S. '999, is a stable product which is readily obtained in very high ( 90%) yields because the substitution is allowed to go to completion. 55 The formaldehyde and nitromethane can be contacted in less than a 3:1 molar ratio provided the reaction product is not isolated but, instead, the product solution is used directly in step two described below and, in turn, the product solution of step 2 is used in step three, as also described below. The formaldehyde and nitromethane can be used in molar ratios of about 2.25 or greater. When used in molar ratios of less than 3, the resultant solution will contain a mixture of di and trihydroxymethyl nitromethane. The product should not be go isolated but, instead, the solution should be directly treated with a ketone or an ether as described below in the presence of an acid (sufficient to neutralize the small amount of base present and to make the solution acidic). The solution containing the products of this procedure should be directly used in the procedure of step three to provide product III. EP 0 348 223 A2 When the molar ratio is greater than 3, the products of steps 1 and 2 may be isolated or the solution may be directly used in the subsequent step. 2. The formed tris(hydroxymethyl)nitromethane (I) is then reacted with a ketone in the presence of a catalytic amount of a strong acid to form the corresponding acetal, the 5-hydroxymethyl-5-nitro-1,3-dioxane which has substitution in the 2 position (II), in good yields. 0 NO- ^CH»OH2 II H+ ^^v>^C C(CHn0H)o + RCR1 I 2 '3 10 NO_ 2 0 R R' 15 (I) (ID Each R and R' can independently be selected from a C1-C10 alkyl, C3-C10 cycloalkyl or aryl (preferably phenyl) 20 group or R and R' can together form a C4-C10 alkylene group, and preferably a C4-C6 alkylene group. The particular identity of R and R' is not critical to this reaction nor to the overall synthesis. Examples of suitable ketones include acetone, methyl ethyl ketone, diethyl ketone, cyclohexanone and the like. The reaction can be run neat using excess ketone as the reaction medium (preferred) or by using an inert solvent in which both compound I and the ketone are soluble. The reaction is catalyzed by the presence of catalytic amounts 25 (normally from about 0.001 to 1 weight percent based on the weight of ketone) of a strong acid, such as a mineral acid (HCI, H2SO4 and the like) or a strong organic acid such as glacial acetic acid, toluene sulfonic acid and the like. The above reaction (2) produces water as a by-product. The water must be removed in order to prevent reversion of the formed acetal back to the ketone and alcohol. When the reaction utilizes a high boiling ketone 30 (having a B.P. higher than water and suitable for separating the water from ketone by distillation), such as cyclohexanone, the water by-product can be removed by azeotropic distillation during the progress of the reaction. When a low boiling ketone, having a boiling point lower than the water, such as acetone, is used the procedure requires the presence of a dessicant, such as boron trifluoride etherate or a molecular sieve which collects water to remove the water as it forms. 35 Although the above reaction utilizes readily attainable and inexpensive reactants, the need to remove the water by-product as it forms may add to the cost of the reaction and the overall synthesis. If such economics presents a factor, the formation of an acetal can be accomplished without the production of water by alternate reactions, as described hereinbelow. 2(A). The tris(hydroxymethyl)nitromethane (I) can be converted to an acetal by reacting it with vinyl ether in 40 the presence of a catalytic amount of strong acid (such as mineral acids, glacial acetic acid and the like) by the following reaction: f2 N^/CH2OH C(CH2OH)3 + H2C = CHOR 50 I Ha 55 R can represent any alkyl, cycloalkyl or aryl group and is preferably a lower alkyl. Examples of suitable vinyl ethers include ethyl vinylether, methyl vinylether and the like. The resultant by-product alcohol does not interfere with the reaction. 2(B). Again, as an alternate means, the desired acetal compound can be provided by reacting the tris(hydroxymethyl)nitromethane (I) with certain gem diethers in the presence of a catalytic amount of a strong 60 acid (such as mineral acid, glacial acetic acid and the like) by the following reaction: 65 EP 0 348 223 A2 NO OR" NO,, CH.OH ^ C(CH2OH)3 + R-C-R' * f OR" 0 0+ R"0H 10 ^R ^R1 The symbols, R and R' are the same as described above with respect to reaction 2 and R" can be any alkyl, preferably a lower alkyl such as methyl, ethyl, propyl and the like.
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